This invention relates to hot melt ink printing on porous substrates such as paper and to methods and apparatus for providing improved hot melt ink printing on such substrates.
Hot melt inks are used in thermal transfer printers and in certain ink jet printers. The characteristics of these inks is that they are solid at room temperature, are liquefied by heating for application, and are resolidified by cooling on the printed substrate. Heretofore, hot melt ink systems having been designed to apply hot melt ink to a porous substrate such as paper at a temperature which is high enough to permit the ink to flow into the substrate before solidifying, but not high enough to cause the ink to pass through the substrate.
In the Spehrley, Jr., et al. Pat. No. 4,751,528, a hot melt ink jet system is described in which the temperature of the platen supporting a paper substrate to which ink is supplied by an ink jet heats the substrate to a selected level which is related to the melting point of the ink so as to permit the ink to spread to a desired extent within the substrate before solidification. However, such heating of paper removes moisture in an uncontrolled manner which causes paper shrinkage and cockle, degrading the image quality and paper handling reliability. Furthermore, maintaining the temperature of the platen at a selected level so as to control spreading of the ink in the substrate presents significant difficulties, for example, because of varying rates of application of the molten ink to the substrate.